1. Field of the Invention
This invention relates to a connection processing method for radiation images and a radiation image processing apparatus for carrying out the method. This invention particularly relates to connection processing for radiation images, which is performed in cases where a radiation image of an object having been recorded on a plurality of stimulable phosphor sheets associated with one another is to be reconstructed.
2. Description of the Prior Art
Recently, as systems capable of obtaining radiation images recorded even when energy intensity of radiation, to which a recording medium is exposed, varies over a wide range, computed radiography systems (CR systems) have widely been used in practice. With the CR systems, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored, thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In the CR systems, stimulable phosphor sheets having various different sizes, such as a 14xe2x80x3xc3x9717xe2x80x3 size, a 14xe2x80x3xc3x9714xe2x80x3 size, a 10xe2x80x3xc3x9712xe2x80x3 size, and a 8xe2x80x3xc3x9710xe2x80x3 size, have heretofore been used in accordance with the objects whose images are to be recorded. However; in the fields of the orthopedic surgery, for the purposes of measuring the degree of bending of the spinal column, and the like, there is a strong demand for the use of a long image ranging from a pattern of the neck to a pattern of the waist as a single image. Therefore, it has been studied to utilize stimulable phosphor sheets which are longer than the aforesaid sizes in a predetermined direction.
However, in cases where the long stimulable phosphor sheets are utilized, designs of radiation image read-out apparatuses for reading out the radiation images from the stimulable phosphor sheets, such as the designs of sheet conveyance paths in the radiation image read-out apparatuses, must be altered markedly so as to be adapted to the long stimulable phosphor sheets. The radiation image read-out apparatuses must thus be designed for the exclusive use for the long stimulable phosphor sheets. Therefore, the problems occur in that the radiation image read-out apparatuses designed for the long stimulable phosphor sheets are disadvantageous in the aspect of cost.
Accordingly, a technique may be utilized, wherein two stimulable phosphor sheets having the conventional sizes are associated with each other to form an apparently long stimulable phosphor sheet, a long image is recorded on the apparently long stimulable phosphor sheet, and thereafter the two stimulable phosphor sheets constituting the apparently long stimulable phosphor sheet are subjected to image read-out operations one after the other. With the technique, the image read-out operations can be performed by utilizing the conventional radiation image read-out apparatus without its design being altered, and the problems described above do not occur.
Also, with the technique described above, three or more stimulable phosphor sheets can be associated with one another to form an;apparently long stimulable phosphor sheet, and a long image of an object can be recorded on the apparently long stimulable phosphor sheet. Also, a plurality of stimulable phosphor sheets can be associated with one another in two axis directions, which are normal to each other, in order to form an apparently wide, long stimulable phosphor sheet, and a wide, long image of an object can be recorded on the apparently wide, long stimulable phosphor sheet. Therefore, the technique described above has good adaptability to objects.
In cases where at least two stimulable phosphor sheets are associated with each other to form an apparently long stimulable phosphor sheet and an image of an object is recorded on the apparently long stimulable phosphor sheet, if the two adjacent stimulable phosphor sheets among the plurality of the stimulable phosphor sheets are considered, the two adjacent stimulable phosphor sheets may be associated with each other such that their edges are in abutment with each other. Alternatively, the two adjacent stimulable phosphor sheets may be associated with each other such that portions of the two sheets overlap each other. However, with the technique wherein the two adjacent stimulable phosphor sheets are associated with each other such that their edges are in abutment with each other, loss of image information will inevitably occurs at the boundary area between the two adjacent stimulable phosphor sheets. With the technique wherein the two adjacent stimulable phosphor sheets are associated with each other such that the portions of the two sheets overlap each other, such loss of image information does not occur.
However, with the technique wherein the two adjacent stimulable phosphor sheets are associated with each other such that the portions of the two sheets overlap each other, each of the two radiation images having been read out from the two stimulable phosphor sheets contains an image pattern of the overlapping area. Therefore, even if the two radiation images having; been read out from the two stimulable phosphor sheets are merely connected with each other without any spacing therebetween, a correct radiation image of the object cannot be reconstructed.
The primary object of the present invention is to provide a connection processing method for radiation images, wherein radiation images, which have been recorded on a plurality of stimulable phosphor sheets associated with one another with portions of adjacent stimulable phosphor sheets overlapping each other, are read out, positions of the read-out radiation images are accurately matched with one another, and a single radiation image is reconstructed from the read-out radiation images.
Another object of the present invention is to provide a radiation image processing apparatus for carrying out the connection processing method for radiation images.
In cases where two adjacent stimulable phosphor sheets are associated with each other such that portions of the two adjacent stimulable phosphor sheets overlap each other, and radiation carrying image information of an object is irradiated to the two adjacent stimulable phosphor sheets, the overlapping area of one of the two adjacent stimulable phosphor sheets, which is located on the side remote from the object, is exposed to the radiation having decayed to a dose smaller than the dose of the radiation irradiated to the other area, which does not overlap the other stimulable phosphor sheet. Therefore, an image pattern of a boundary line due to a difference in image density is formed between the overlapping area and the non-overlapping area of the stimulable phosphor sheet located on the side remote from the object. A first connection processing method for radiation images and a first radiation image processing apparatus in accordance with the present invention are characterized by detecting the thus formed boundary line image pattern and matching positions of two radiation images with each other by the utilization of the detected boundary line image pattern.
Specifically, the present invention provides a first connection processing method for radiation images, in which a single radiation image of an object is recorded on a plurality of stimulable phosphor sheets associated with one another such that portions of two adjacent stimulable phosphor sheets overlap each other, and in which connection processing is performed on a plurality of radiation images having been read out from the plurality of the stimulable phosphor sheets respectively, such that the single radiation image of the object is reconstructed from the plurality of the read-out radiation images, the method comprising the steps of:
i) detecting a boundary line image pattern of an overlapping region of a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object, which first stimulable phosphor sheet is one of the two adjacent stimulable phosphor sheets, the overlapping region corresponding to an overlapping area of the first stimulable phosphor sheet located on the side remote from the object, which overlapping area overlaps an overlapping area of the other second stimulable phosphor sheet located on the side close to the object, and
ii) performing matching of positions of two radiation images, which have been read out from the two adjacent stimulable phosphor sheets respectively, with each other in accordance with the position of the detected boundary line image pattern in the first radiation image, which has been read out from the first stimulable phosphor sheet located on the side remote from the object, and the position of an edge of an overlapping region of a second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, the overlapping region of the second radiation image corresponding to the overlapping area of the second stimulable phosphor sheet.
The term xe2x80x9cside remote from an objectxe2x80x9d as used herein means the side remote from the object at the area at which the two adjacent stimulable phosphor sheets overlap each other.
Of the two adjacent stimulable phosphor sheets, the stimulable phosphor sheet remote from the object at the area at which the two adjacent stimulable phosphor sheets overlap each other is herein referred to as the first stimulable phosphor sheet, and the stimulable phosphor sheet close to the object at the area at which the two adjacent stimulable phosphor sheets overlap each other is herein referred to as the second stimulable phosphor sheet. Also, the radiation image having been read out from the first stimulable phosphor sheet is herein referred to as the first radiation image, and the radiation image having been read out from the second stimulable phosphor sheet is herein referred to as the second radiation image.
The term xe2x80x9csingle radiation image of an object being recordedxe2x80x9d as used herein means that one radiation image of the object containing image patterns of backgrounds of the object is recorded, and does not mean that the image of only the object is recorded.
In the first connection processing method for radiation images in accordance with the present invention, the detection of the boundary line image pattern may be performed by performing edge detection processing (such as differentiation processing) on a radiation image signal, which represents the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object.
Also, in the first connection processing method for radiation images in accordance with the present invention, the matching of positions of the two radiation images, which have been read out from the two adjacent stimulable phosphor sheets respectively, with each other is performed in accordance with the position of the boundary line image pattern in the first radiation image, which has been read out from the first stimulable phosphor sheet located on the side remote from the object, and the position of the edge of the overlapping region of the second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object. Specifically, in cases where the second radiation image can be read out from the second stimulable phosphor sheet such that the image information having been recorded at the edge area of the second stimulable phosphor sheet can be read out perfectly, the matching of positions of the two radiation images with each other may be performed such that the position of the edge of the overlapping region of the second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, coincides with the position of the boundary line image pattern in the first radiation image, which has been read out from the first stimulable phosphor sheet located on the side remote from the object. In cases where the image information having been recorded at the edge area of the second stimulable phosphor sheet cannot be read out perfectly, the matching of positions of the two radiation images with each other may be performed such that the position of the edge of the second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, coincides with a position in the first radiation image, which position is shifted from the position of the boundary line image pattern in the first radiation image into the overlapping region of the first radiation image by the distance corresponding to the length of the unreadable edge area of the second stimulable phosphor sheet.
Further, in the first connection processing method for radiation images in accordance with the present invention, the connection processing on the radiation images having been read out from the two adjacent stimulable phosphor sheets should preferably be performed such that image signal components of a radiation image signal representing the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, which image signal components correspond to the overlapping region of the second radiation image, are overwritten upon the image signal components of a radiation image signal representing the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, which image signal components correspond to the overlapping region of the first radiation image. As described above, the overlapping area of the first stimulable phosphor sheet located on the side remote from the object is exposed to a smaller dose of radiation than the dose of the radiation irradiated to the non-overlapping area of the first stimulable phosphor sheet. Therefore, if the image signal components of the radiation image signal representing the first radiation image, which correspond to the overlapping region of the first radiation image, are utilized in the connection processing, a difference in image density will occur at the area at which the two radiation images are connected. However, in cases where the image signal components of the radiation image signal representing the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, which correspond to the overlapping region of the second radiation image, are utilized in the connection processing, no difference in image density occurs at the area at which the two radiation images are connected.
However, in cases where the image information having been recorded at the edge area of the second stimulable phosphor sheet cannot be read out perfectly, the matching of positions of the two radiation images with each other is performed such that the position of the edge of the second radiation image, which has been read out from the second stimulable phosphor sheet, coincides with the position in the first radiation image, which position is shifted from the position of the boundary line image pattern in the first radiation image into the overlapping region of the first radiation image by the distance corresponding to the length of the unreadable edge area of the second stimulable phosphor sheet. Therefore, as for the unreadable edge area of the second stimulable phosphor sheet, the image signal components of the radiation image signal representing the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object must be utilized. In such cases, in the radiation image reconstructed after the connection processing, the image density of the image region corresponding to the unreadable edge area becomes lower than the image density of the other image region. As for the image region having the image density lower than the image density of the other image region, correction of the image density, e.g. uniform shifting of the image density values to large values, may be performed such that the image density of the image region approximately coincides with the image density of the non-overlapping region of the first radiation image.
In cases where the correction of the image density is made, the connection processing need not necessarily be limited to the aforesaid processing, wherein the image signal components of the radiation image signal representing the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, which correspond to the overlapping region of the second radiation image, are overwritten upon the image signal components of the radiation image signal representing the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, which correspond to the overlapping region of the first radiation image. Conversely, the connection processing may be performed such that the image signal components of the radiation image signal representing the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, which correspond to the overlapping region of the first radiation image, are overwritten upon the image signal components of the radiation image signal representing the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, which correspond to the overlapping region of the second radiation image. This is because a substantial difference in time required to perform the correction processing does not occur between when the image density correction is made with respect to only the part (the unreadable edge area) of the overlapping region and when the image density correction is performed with respect to the entire overlapping region. However, since the image information at the overlapping region of the first radiation image has been recorded with a smaller dose of radiation than the dose of radiation with which the second radiation image has been recorded, graininess (noise) characteristics of the overlapping region of the first radiation image are worse than the graininess (noise) characteristics of the second radiation image. Therefore, as for the overlapping region, the image signal components of the radiation image signal representing the second radiation image should preferably be utilized in the connection processing.
In the image recording operation for recording an image of an object on a stimulable phosphor sheet, radiation is produced by a radiation source as a divergent beam and is irradiated to the object. Therefore, the size of the image of the object recorded on the first stimulable phosphor sheet located on the side remote from the object and the size of the image of the object recorded on the second stimulable phosphor sheet are slightly different from each other. The image recorded on the first stimulable phosphor sheet located on the side remote from the object is larger than the image recorded on the second stimulable phosphor sheet located on the side close to the object. Therefore, in cases where there is the risk of the difference between the sizes of the two radiation images, which are to be subjected to the connection processing, adversely affecting the image quality of the reconstructed image, image size enlargement or reduction processing may be performed on the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object and/or the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, such that the sizes of the first radiation image and the second radiation image coincide with each other.
In cases where it cannot be specified previously which one of the two radiation images having been read out from the two adjacent stimulable phosphor sheets is the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object or is the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, the technique described below should preferably be employed for achieving automatic processing. Specifically, edge detection processing is performed on each of two radiation image signals representing the two radiation images having been read out from the two adjacent stimulable phosphor sheets, and it is specified, in accordance with the results of the edge detection processing, which one of the two radiation images is the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object or is the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object.
In cases where the technique described above is employed and the number of the stimulable phosphor sheets associated with one another is two, the boundary line image pattern appears only in either one of the two radiation images. Therefore, with the edge detection processing, it can be specified that the radiation image, from which the boundary line image pattern has been detected, is the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, and that the radiation image, from which no boundary line image pattern has been detected with the edge detection processing, is the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object. In cases where the number of the stimulable phosphor sheets associated with one another is larger than two, merely in accordance with the presence or absence of the boundary line image pattern, it cannot be specified which one of the radiation images having been read out from the stimulable phosphor sheets is a radiation image having been read out from a stimulable phosphor sheet located on the side remote from the object or is a radiation image having been read out from a stimulable phosphor sheet located on the side close to the object. However, the overlapping area, at which two adjacent stimulable phosphor sheets overlap each other, can be limited to a restricted area of each stimulable phosphor sheet. Therefore, within the restricted area, the boundary line image pattern appears only in either one of the two radiation images. Accordingly, with the edge detection processing, it can be specified that the radiation image, from which the boundary line image pattern has been detected, is the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, and that the radiation image, from which no boundary line image pattern has been detected with the edge detection processing, is the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object.
The present invention also provides a first radiation image processing apparatus for carrying out the first connection processing method for radiation images in accordance with the present invention. Specifically, the present invention also provides a first radiation image processing apparatus, in which a single radiation image of an object is recorded on a plurality of stimulable phosphor sheets associated with one another such that portions of two adjacent stimulable phosphor sheets overlap each other, and in which connection processing means is provided for performing connection processing on a plurality of radiation images having been read out from the plurality of the stimulable phosphor sheets respectively, such that the single radiation image of the object is reconstructed from the plurality of the read-out radiation images, the apparatus comprising:
i) boundary line image pattern detecting means for detecting a boundary line image pattern of an overlapping region of a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object, which first stimulable phosphor sheet is one of the two adjacent stimulable phosphor sheets, the overlapping region corresponding to an overlapping area of the first stimulable phosphor sheet located on the side remote from the object, which overlapping area overlaps an overlapping area of the other second stimulable phosphor sheet located on the side close to the object, and
ii) position matching means for performing matching of positions of two radiation images, which have been read out from the two adjacent stimulable phosphor sheets respectively, with each other in accordance with the position of the detected boundary line image pattern in the first radiation image, which has been read out from the first stimulable phosphor sheet located on the side remote from the object, and the position of an edge of an overlapping region of a second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, the overlapping region of the second radiation image corresponding to the overlapping area of the second stimulable phosphor sheet,
wherein the connection processing means performs the connection processing on the two radiation images, the positions of which have been matched with each other by the position matching means.
In the first radiation image processing apparatus in accordance with the present invention, the boundary line image pattern detecting means may detect the boundary line image pattern by performing edge detection processing on a radiation image signal, which represents the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object.
Also, in the first radiation image processing apparatus in accordance with the present invention, in cases where the second radiation image can be read out from the second stimulable phosphor sheet such that the image information having been recorded at the edge area of the second stimulable phosphor sheet can be read out perfectly, the position matching means may perform the matching of positions of the two radiation images with each other such that the position of the edge of the overlapping region of the second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, coincides with the position of the boundary line image pattern in the first radiation image, which has been read out from the first stimulable phosphor sheet located on the side remote from the object. In cases where the image information having been recorded at the edge area of the second stimulable phosphor sheet cannot be read out perfectly, the position matching means may perform the matching of positions of the two radiation images with each other such that the position of the edge of the second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, coincides with a position in the first radiation image, which position is shifted from the position of the boundary line image pattern in the first radiation image into the overlapping region of the first radiation image by the distance corresponding to the length of the unreadable edge area of the second stimulable phosphor sheet.
Further, in the first radiation image processing apparatus in accordance with the present invention, the connection processing means should preferably perform the connection processing on the radiation images having been read out from the two adjacent stimulable phosphor sheets such that image signal components of a radiation image signal representing the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, which image signal components correspond to the overlapping region of the second radiation image, are overwritten upon the image signal components of a radiation image signal representing the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, which image signal components correspond to the overlapping region of the first radiation image.
In the image recording operation for recording an image of an object on a stimulable phosphor sheet, radiation is produced by a radiation source as a divergent beam and is irradiated to the object. Therefore, the size of the image of the object recorded on the first stimulable phosphor sheet located on the side remote from the object and the size of the image of the object recorded on the second stimulable phosphor sheet are slightly different from each other. The image recorded on the first stimulable phosphor sheet located on the side remote from the object is larger than the image recorded on the second stimulable phosphor sheet located on the side close to the object. Therefore, in cases where there is the risk of the difference between the sizes of the two radiation images, which are to be subjected to the connection processing, adversely affecting the image quality of the reconstructed image, image size enlargement or reduction processing may be performed on the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object and/or the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, such that the sizes of the first radiation image and the second radiation image coincide with each other.
In cases where it cannot be specified previously which one of the two radiation images having been read out from the two adjacent stimulable phosphor sheets is the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object or is the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, the first radiation image processing apparatus in accordance with the present invention should preferably further comprise radiation image specifying means for performing the technique described below for achieving automatic processing. Specifically, the first radiation image processing apparatus in accordance with the present invention should preferably further comprise radiation image specifying means for:
performing edge detection processing on each of two radiation image signals representing the two radiation images having been read out from the two adjacent stimulable phosphor sheets, and
specifying, in accordance with the results of the edge detection processing, which one of the two radiation images is the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object or is the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object.
A second connection processing method for radiation images and a second radiation image processing apparatus in accordance with the present invention, in which connection processing is performed on two radiation images having been read out respectively from two adjacent stimulable phosphor sheets, are characterized by performing template matching with respect to overlapping regions of the two radiation images, which overlapping regions correspond to overlapping areas of the two stimulable phosphor sheets, and thereby matching the positions of the two radiation images with each other.
Specifically, the present invention further provides a second connection processing method for radiation images, in which a single radiation image of an object is recorded on a plurality of stimulable phosphor sheets associated with one another such that portions of two adjacent stimulable phosphor sheets overlap each other, and in which connection processing is performed on a plurality of radiation images having been read out from the plurality of the stimulable phosphor sheets respectively, such that the single radiation image of the object is reconstructed from the plurality of the read-out radiation images, the method comprising the steps of:
i) setting at least one subregion within an overlapping region of a radiation image, which has been read out from either one of the two adjacent stimulable phosphor sheets, as a template, the overlapping region corresponding to an overlapping area of the one stimulable phosphor sheet, which overlapping area overlaps an overlapping area of the other stimulable phosphor sheet,
ii) searching a subregion, which coincides with the template, within an overlapping region of a radiation image, which has been read out from the other stimulable phosphor sheet, the overlapping region corresponding to the overlapping area of the other stimulable phosphor sheet, and
iii) performing matching of positions of the two radiation images with each other such that the thus searched subregion and the template coincide with each other.
In the second connection processing method for radiation images in accordance with the present invention, only one template may be set in the radiation image, which has been read out from either one of the two adjacent stimulable phosphor sheets. Alternatively, a plurality of templates may be set in the radiation image. Such that the reliability of the position matching may be kept high, a plurality of templates should preferably be set. In cases where a plurality of templates are set, different subregions within the overlapping region of the radiation image, which has been read out from the one stimulable phosphor sheet, may be set as the templates. Also, when a plurality of subregions, which coincide with the templates, are searched within the overlapping region of the radiation image, which has been read out from the other stimulable phosphor sheet, the plurality of the subregions may be searched such that the relationship among the positions of the plurality of the templates in the radiation image having been read from the one stimulable phosphor sheet is kept, and such that the plurality of the templates simultaneously coincide with the plurality of the subregions. Further, when the matching of positions of the two radiation images with each other is performed, the position matching may be performed such that all of the templates in the one radiation image and all of the subregions having been searched in the other radiation image simultaneously coincide with each other.
The template may be set within the overlapping region of the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, and the subregion coinciding with the template may be searched within the overlapping region of the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object. Conversely, the template may be set within the overlapping region of the second radiation image, and the subregion coinciding with the template may be searched within the overlapping region of the first radiation image. In cases where a feature image pattern is automatically detected within the overlapping region and the template is set automatically in accordance with the detected feature image pattern as will be described later, since the image information within the overlapping region of the second radiation image has been recorded with a larger dose of radiation than that delivered to the overlapping area of the first stimulable phosphor sheet, the feature image pattern can be detected more accurately from the second radiation image than from the first radiation image. Therefore, the template should preferably be set within the overlapping region of the second radiation image.
In the second connection processing method for radiation images in accordance with the present invention, such that the processing may be kept simple, the overlapping region of the radiation image having been read out from the one stimulable phosphor sheet and the overlapping region of the radiation image having been read out from the other stimulable phosphor sheet should preferably be detected by the steps of:
detecting a boundary line image pattern of an overlapping region of a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object, which first stimulable phosphor sheet is one of the two adjacent stimulable phosphor sheets, the overlapping region corresponding to an overlapping area of the first stimulable phosphor sheet located on the side remote from the object, which overlapping area overlaps an overlapping area of the other second stimulable phosphor sheet located on the side close to the object, and
detecting the overlapping region of the first radiation image and an overlapping region of a second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, in accordance with the detected boundary line image pattern. The detection of the boundary line image pattern may be performed by performing edge detection processing (such as differentiation processing) on a radiation image signal, which represents the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object. One of techniques for detecting the boundary line image pattern will be described hereinbelow.
Specifically, at the overlapping area, the second stimulable phosphor sheet covers the first stimulable phosphor sheet. Therefore, the overlapping area of the first stimulable phosphor sheet is exposed to a smaller dose of radiation than that delivered to the non-overlapping area of the first stimulable phosphor sheet. Accordingly, in the first radiation image having been read out from the first stimulable phosphor sheet, a difference in image density occurs between the overlapping region, which corresponds to the overlapping area of the first stimulable phosphor sheet, and the non-overlapping region, which corresponds to the non-overlapping area of the first stimulable phosphor sheet. (The term xe2x80x9cimage densityxe2x80x9d as used herein means the gray level, the luminous level, and the like, in an image having gradation. The term xe2x80x9cimage densityxe2x80x9d as used herein also means the luminance of an image displayed on a display device, such as a cathode ray tube (CRT) display device. In cases where the radiation image is expressed as an image signal, the term xe2x80x9cimage densityxe2x80x9d as used herein means the image signal value.) As a result, the boundary line image pattern due to the difference in image density is formed between the overlapping region and the non-overlapping region of the first radiation image. By way of example, the boundary line image pattern can be detected by performing the edge detection processing, such as the differentiation processing, on the image signal representing the first radiation image and along the direction of association of the two stimulable phosphor sheets.
After the boundary line image pattern in the first radiation image has been detected in the manner described above, the region ranging from the boundary line image pattern to the edge of the first radiation image on the side of the overlapping region can be recognized as the overlapping region of the first radiation image. As for the second radiation image, the region extending from the edge of the second radiation image on the side of the overlapping region inwardly by a length, which is equal to the length between the boundary line image pattern in the first radiation image and the edge of the first radiation image on the side of the overlapping region, may be recognized as the overlapping region of the second radiation image. In cases where the second radiation image can be read out from the second stimulable phosphor sheet such that the image information having been recorded at the edge area of the second stimulable phosphor sheet can be read out perfectly, as described above, the overlapping region of the first radiation image and the overlapping region of the second radiation image coincides perfectly with each other. However, in cases where the image information having been recorded at the edge area of the second stimulable phosphor sheet cannot be read out perfectly, the position spaced by the length of the unreadable edge area from the edge of the second stimulable phosphor sheet constitutes the edge of the second radiation image having thus been read out. Therefore, the overlapping region of the first radiation image and the overlapping region of the second radiation image do not necessarily coincide with each other. It often occurs that the overlapping region of the second radiation image becomes narrower by the length of the unreadable edge area than the overlapping region of the first radiation image. Therefore, the range having the length resulting from subtraction of the length, which is equal to the length of the unreadable edge area, from the edge of the second radiation image on the overlapping region side may be recognized as the overlapping region.
One of various subregions within the overlapping region of either one of the radiation images may be set as the template in the overlapping region. In order for the template matching to be performed accurately, a subregion containing a feature image pattern within the overlapping region of the radiation image, which has been read out from the one stimulable phosphor sheet, should preferably be set as the template.
By way of example, in cases where position matching markers formed from a material having a low radiation transmittance are located at the overlapping areas of the two adjacent stimulable phosphor sheets and the image recording operation is performed in this state, the marker image patterns appearing within each of the overlapping regions of the two radiation images may be employed as the feature image pattern. Alternatively, a bone image pattern having a characteristic shape (particularly, an edge area of the bone image pattern), an image pattern of ribs intersecting with each other (particularly, an edge area of the image pattern), a lung image pattern (particularly, an edge area of the lung image pattern), or the like, which appears in the object image, may be employed as the feature image pattern.
The feature image pattern within the overlapping region may be detected automatically in accordance with the image information stored within the overlapping region (i.e., the corresponding image signal components of the radiation image signal representing the radiation image). Alternatively, the operator may make a judgment as to the feature image pattern and may thereby detect it. Also, the template may be set automatically in accordance with the detected feature image pattern. Alternatively, the template may be set manually by the operator. Such that the labor and time required to perform the detection and the setting may be reduced, the detection of the feature image pattern and the setting of the template should preferably be performed automatically. In cases where the feature image pattern is to be detected automatically, a detection algorithm for detecting a predetermined image pattern as the feature image pattern may be stored previously, and the feature image pattern may be detected automatically in accordance with the detection algorithm. In cases where the template is to be set automatically, a template setting algorithm for setting a local area limited region, which has a predetermined shape (a rectangular shape, a circular shape, or the like) and contains a neighboring region of the detected feature image pattern, as the template may be stored previously, and the template may be set automatically in accordance with the template setting algorithm.
Further, in the second connection processing method for radiation images in accordance with the present invention, the connection processing on the radiation images having been read out from the two adjacent stimulable phosphor sheets should preferably be performed such that image signal components of a radiation image signal representing a second radiation image having been read out from a second stimulable phosphor sheet located on the side close to the object, which image signal components correspond to the overlapping region of the second radiation image, are overwritten upon the image signal components of a radiation image signal representing a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object, which image signal components correspond to the overlapping region of the first radiation image. As described above, the overlapping area of the first stimulable phosphor sheet located on the side remote from the object is exposed to a smaller dose of radiation than the dose of the radiation irradiated to the non-overlapping area of the first stimulable phosphor sheet. Therefore, if the image signal components of the radiation image signal representing the first radiation image, which correspond to the overlapping region of the first radiation image, are utilized in the connection processing, a difference in image density will occur at the area at which the two radiation images are connected. However, in cases where the image signal components of the radiation image signal representing the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, which image signal components correspond to the overlapping region of the second radiation image, are utilized in the connection processing, no difference in image density occurs at the area at which the two radiation images are connected.
However, in cases where the image information having been recorded at the edge area of the second stimulable phosphor sheet cannot be read out perfectly, when the matching of positions of the two radiation images with each other is performed, the position of the edge of the second radiation image, which has been read out from the second stimulable phosphor sheet, does not coincide with the position of the boundary line image pattern in the first radiation image. (Specifically, the edge of the second radiation image is located at the position in the first radiation image, which position is shifted from the position of the boundary line image pattern in the first radiation image into the overlapping region of the first radiation image by the distance corresponding to the length of the unreadable edge area of the second stimulable phosphor sheet.) Therefore, as for the unreadable edge area of the second stimulable phosphor sheet, the image signal components of the radiation image signal representing the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object must be utilized. In such cases, in the radiation image reconstructed after the connection processing, the image density of the image region corresponding to the unreadable edge area becomes lower than the image density of the other image region. As for the image region having the image density lower than the image density of the other image region, correction of the image density, e.g. uniform shifting of the image density values to large values, may be performed such that the image density of the image region approximately coincides with the image density of the non-overlapping region of the first radiation image.
As in the first connection processing method for radiation images in accordance with the present invention, in cases where the correction of the image density is made, the connection processing need not necessarily be limited to the aforesaid processing, wherein the image signal components of the radiation image signal representing the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, which image signal components correspond to the overlapping region of the second radiation image, are overwritten upon the image signal components of the radiation image signal representing the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, which image signal components correspond to the overlapping region of the first radiation image. Conversely, the connection processing may be performed such that the image signal components of the radiation image signal representing the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, which image signal components correspond to the overlapping region of the first radiation image, are overwritten upon the image signal components of the radiation image signal representing the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, which image signal components correspond to the overlapping region of the second radiation image. However, as described above, as for the overlapping region, the image signal components of the radiation image signal representing the second radiation image should preferably be utilized in the connection processing.
In the image recording operation for recording an image of an object on a stimulable phosphor sheet, radiation is produced by a radiation source as a divergent beam and is irradiated to the object. Therefore, the size of the image of the object recorded on the first stimulable phosphor sheet located on the side remote from the object and the size of the image of the object recorded on the second stimulable phosphor sheet are slightly different from each other. The size of the first radiation image becomes larger than the size of the second radiation image. Therefore, in cases where there is the risk of the difference between the sizes of the two radiation images, which are to be subjected to the connection processing, adversely affecting the image quality of the reconstructed image, image size enlargement or reduction processing may be performed on the first radiation image and/or the second radiation image, such that the sizes of the first radiation image and the second radiation image coincide with each other.
Also, as in the first connection processing method for radiation images in accordance with the present invention, in cases where it cannot be specified previously which one of the two radiation images having been read out from the two adjacent stimulable phosphor sheets is the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object or is the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, the technique described below should preferably be employed for achieving automatic processing. Specifically, edge detection processing is performed on each of two radiation image signals representing the two radiation images having been read out from the two adjacent stimulable phosphor sheets, and it is specified, in accordance with the results of the edge detection processing, which one of the two radiation images is a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object or is a second radiation image having been read out from a second stimulable phosphor sheet located on the side close to the object.
The present invention still further provides a second radiation image processing apparatus for carrying out the second connection processing method for radiation images in accordance with the present invention. Specifically, the present invention still further provides a second radiation image processing apparatus, in which a single radiation image of an object is recorded on a plurality of stimulable phosphor sheets associated with one another such that portions of two adjacent stimulable phosphor sheets overlap each other, and in which connection processing means is provided for performing connection processing on a plurality of radiation images having been read out from the plurality of the stimulable phosphor sheets respectively, such that the single radiation image of the object is reconstructed from the plurality of the read-out radiation images, the apparatus comprising:
i) template setting means for setting at least one subregion within an overlapping region of a radiation image, which has been read out from either one of the two adjacent stimulable phosphor sheets, as a template, the overlapping region corresponding to an overlapping area of the one stimulable phosphor sheet, which overlapping area overlaps an overlapping area of the other stimulable phosphor sheet,
ii) template matching means for searching a subregion, which coincides with the template, within an overlapping region of a radiation image, which has been read out from the other stimulable phosphor sheet, the overlapping region corresponding to the overlapping area of the other stimulable phosphor sheet, and
iii) position matching means for performing matching of positions of the two radiation images with each other such that the thus searched subregion and the template coincide with each other.
In the second radiation image processing apparatus in accordance with the present invention, the template setting means may set only one template in the radiation image, which has been read out from either one of the two adjacent stimulable phosphor sheets. Alternatively, the template setting means may set a plurality of templates in the radiation image. Such that the reliability of the position matching may be kept high, a plurality of templates should preferably be set.
The second radiation image processing apparatus in accordance with the present invention should preferably further comprise overlapping region detecting means for:
detecting a boundary line image pattern of an overlapping region of a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object, which first stimulable phosphor sheet is one of the two adjacent stimulable phosphor sheets, the overlapping region corresponding to an overlapping area of the first stimulable phosphor sheet located on the side remote from the object, which overlapping area overlaps an overlapping area of the other second stimulable phosphor sheet located on the side close to the object, and
detecting the overlapping region of the first radiation image and an overlapping region of a second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, in accordance with the detected boundary line image pattern. In such cases, the overlapping region detecting means should preferably detect the boundary line image pattern by performing edge detection processing on a radiation image signal, which represents the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object.
The template setting means should preferably set a subregion containing a feature image pattern within the overlapping region of the radiation image, which has been read out from the one stimulable phosphor sheet, as the template.
Such that the labor and time required to perform the detection and the setting may be reduced, the template setting means should preferably be constituted such that it automatically detects the feature image pattern (in accordance with image information stored within the overlapping region of the radiation image, which has been read out from the one stimulable phosphor sheet) and automatically sets the template in accordance with the detected feature image pattern.
Also, in the second radiation image processing apparatus in accordance with the present invention, the connection processing means should preferably perform the connection processing on the radiation images having been read out from the two adjacent stimulable phosphor sheets such that image signal components of a radiation image signal representing a second radiation image having been read out from a second stimulable phosphor sheet located on the side close to the object, which image signal components correspond to the overlapping region of the second radiation image, are overwritten upon the image signal components of a radiation image signal representing a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object, which image signal components correspond to the overlapping region of the first radiation image. In such cases, a radiation image having uniform image density can be obtained from the connection processing.
Further, in cases where it cannot be specified previously which one of the two radiation images having been read out from the two adjacent stimulable phosphor sheets is the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object or is the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, the second radiation image processing apparatus in accordance with the present invention should preferably further comprise radiation image specifying means for performing the technique described below for achieving automatic processing. Specifically, the second radiation image processing apparatus in accordance with the present invention should preferably further comprise radiation image specifying means for:
performing edge detection processing on each of two radiation image signals representing the two radiation images having been read out from the two adjacent stimulable phosphor sheets, and
specifying, in accordance with the results of the edge detection processing, which one of the two radiation images is a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object or is a second radiation image having been read out from a second stimulable phosphor sheet located on the side close to the object.
In cases where two adjacent stimulable phosphor sheets are associated with each other such that portions of the two adjacent stimulable phosphor sheets overlap each other, and radiation carrying image information of an object is irradiated to the two adjacent stimulable phosphor sheets, the overlapping area of: the first stimulable phosphor sheet, which is located on the side remote from the object, is exposed to the radiation having decayed to a dose smaller than the dose of the radiation irradiated to the other area, which does not overlap the second stimulable phosphor sheet. Therefore, a difference in image density occurs and the image pattern of the boundary line is formed between the overlapping area and the non-overlapping area of the first stimulable phosphor sheet located on the side remote from the object. With the first connection processing method for radiation images and the first radiation image processing apparatus in accordance with the present invention, the thus formed boundary line image pattern is detected, and the overlapping areas of the two adjacent stimulable phosphor sheets can be specified. Also, the connection processing is performed on the two radiation images, which have been read out from the two adjacent stimulable phosphor sheets respectively, in accordance with the position of the detected boundary line image pattern in the first radiation image, which has been read out from the first stimulable phosphor sheet located on the side remote from the object, and the position of the edge of the overlapping region of the second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object. Therefore, the two radiation images can be connected with each other such that their overlapping regions may overlap each other. Further, even if markers for position matching are not used in the image recording operation, the positions of the radiation images can be accurately matched with each other, and a single connected radiation image can thus be reconstructed from the radiation images.
With the second connection processing method for radiation images and the second radiation image processing apparatus in accordance with the present invention, at least one subregion within the overlapping region of the radiation image, which has been read out from either one of the two adjacent stimulable phosphor sheets, is set as the template, the overlapping region corresponding to the overlapping area of the one stimulable phosphor sheet, which overlapping area overlaps the overlapping area of the other stimulable phosphor sheet. Also, the subregion, which coincides with the template, is searched within the overlapping region of the radiation image, which has been read out from the other stimulable phosphor sheet, the overlapping region corresponding to the overlapping area of the other stimulable phosphor sheet. Thereafter, the matching of positions of the two radiation images with each other is performed such that the thus searched subregion and the template coincide with each other. Therefore, the advantages over the technique, wherein the two adjacent radiation images are merely connected with each other such that any gap may not occur therebetween, can be obtained in that the positions of the two radiation images can be accurately matched with each other and, as a result, a correct radiation image can be reconstructed.
Also, with the second connection processing method for radiation images and the second radiation image processing apparatus in accordance with the present invention, the template matching is performed within the limited region, i.e. within the overlapping region. Therefore, the time required to perform the template matching processing can be kept short. Further, since the template matching is performed within the narrow region, the accuracy with which the template matching is performed can be kept high.